Advancements in micromachining and other microfabrication techniques and processes have enabled the fabrication of a wide variety of MicroElectroMechanical Sensors (MEMS) and other such devices. These devices include moving rotors, gears, switches, accelerometers, miniaturized sensors, actuator systems, and other such structures.
One promising application of MEMS technology is in the area of inertial sensors. Inertial sensors operate by sensing displacements of a proof mass mounted on a compliant suspension. The proof mass displacement ΔX is related to the mass (m) of the proof mass, the acceleration (a), and the spring constant (k) of the compliant suspension as shown in EQUATION 1:ΔX=ma/k  (EQUATION 1)
The sensitivity of the inertial sensor is limited by the ability of the device to accurately sense displacements of the proof mass. Typically, as accelerations become small, the resulting displacements of the proof mass also become small, and it becomes increasingly difficult for the inertial sensor to accurately sense the displacement and thereby accurately register the acceleration.
The sensitivity of the inertial sensor may be increased by decreasing the spring constant (k), since this will result in a larger displacement of the proof mass. However, as the spring constant is decreased, the sensor becomes increasingly prone to vertical stiction. Stiction, which refers to the phenomenon in which a moving component of a MEMS device adheres to an adjacent surface, typically occurs when surface adhesion forces between the component and the adjacent surface are higher than the mechanical restoring force of the micro-structure. These surface adhesion forces may arise from capillary forces, electrostatic attraction, or direct chemical bonding. In a MEMS device such as an inertial sensor, vertical stiction can cause the device to malfunction. Hence, improvements in the sensitivity of an inertial sensor through reduction in the spring constant alone are limited by the consequent reductions in device reliability.
As suggested by EQUATION I above, the sensitivity of the inertial sensor may also be increased by increasing the mass of the proof mass. However, there are practical limitations to the improvements achievable in the sensitivity of an inertial sensor by increasing the proof mass. In particular, in conventional MEMS devices, the proof mass is constructed out of silicon or other semiconductor materials. Consequently, it is difficult to substantially increase the weight of the proof mass without also substantially increasing the size of the sensor, a result which is very undesirable in light of the current need in the art for further miniaturization of these devices.
There is thus a need in the art for a method for increasing the sensitivity of inertial sensors or other MEMS devices, without adversely affecting device reliability. There is also a need in the art for devices so made. These and other needs are met by the methodologies and devices disclosed herein and hereinafter described.